Interesting point, Amaunet. There are many creator Gods of note. For example, we have:
Nun, Re (Ra), Ptah, Khnum, Atum, Asar (Osiris), Ptah/Asar or Serapis, Heru, Amun, Heru Ur, Hapi, the Aten. In all, I believe there are at least ten "families" of Netjeru. Asar was an excellent first guess, as he is all encompassing, dealing with life, procreation, fertility and death as well. Not all creator gods had death as part of their itinerary.

Some of course (Anpu, Anpu/Upuaut, Sokar, Imset, Haapi, Duamutef, Kebehsenuef, etc. ) were more about death or rebirth after death, but that isn't the question.

We are searching for one, "that represents the entire spectrum of life from birth to death and the transitional point that moves between them."

My first response, is again asking a question (or two) of the questioner. Taysat Wesir, you are not looking for a "Creator" god or a "Death" god, specifically, correct?

Are we looking for Osiris(Asar)/Ptah/Apis; "Serapis," or is he too recent? Your hint mentions the Middle Kingdom.